The phenomenon of thermoelectric cooling has been known since its discovery in 1834 by J. C. A. Peltier. Briefly, a junction between two dissimilar conductors will absorb or evolve heat when a current of respectively one direction or the other flows through it. In the cooling direction, the amount of heat absorbed per unit electric current per unit time is, roughly speaking, given by the product TS where T is the absolute temperature and S is the Seebeck coefficient or “thermopower”. The Seebeck coefficient, which is different for different materials and may be temperature-dependent, is defined as the entropy flux per unit charge at the Fermi level. A useful but less rigorous definition is the rate of change of electrical potential with temperature, where the potential is the steady-state open-circuit voltage between the ends of a conductor to which a temperature gradient has been applied, the temperature variable is the difference in temperature between the two ends, and the sign of S is positive when the potential becomes more negative with rising temperature.
The efficiency of a thermoelectric material is closely related to the dimensionless figure of merit ZT, where Z=S2σ/κ, and σ and κ are respectively the electrical and thermal conductivities of the material. The efficiencies of known thermoelectric materials are currently much too low for thermoelectric heaters and coolers to serve as a practical substitute for mechanical heat pumps and refrigeration systems. According to one rule of thumb, ZT would have to be greater than 3 before such substitutions could become practical. Typical values for the ZT of high-performing thermoelectric materials are in the range 0.5-2.0, with values as high as 2.4 reported for specially structured materials.
In the quest for more efficient thermoelectric cooling, there have been various attempts to modify material properties in order to increase ZT. Because it has proven difficult to modify S, many published attempts have aimed to reduce the thermal conductivity κ. This approach, however, has encountered fundamental limitations because the thermal and electrical conductivities are closely interrelated. In particular, efforts to reduce κ by incoherent phonon scattering have often also led to the unintentional scattering of electrons, which concomitantly reduces σ and therefore nullifies some or all of the advantage gained by reducing κ.
Hence there remains a need for new material systems with properties that lead to still greater thermoelectric efficiency.